Method for the preparation of Ag/C nanocomposite films by laser-induced carbonization of alkane

ABSTRACT

Ag/C crystalline nanocomposite films and a method of forming the films with controllable Ag/C molar ratios using a concurrent excimer laser-induced ablation of a silver target and a hydrocarbon gas under a vacuum atmosphere. Metal/Carbon nanocomposites prepared by concurrent irradiation of a metal target, in the presence of a hydrocarbon gas, during an excimer laser induced process.

CROSS-REFERENCE TO PROVISIONAL APPLICATION

This application claims priority to U.S. Provisional Application No.62/110,189, filed Jan. 30, 2015; the disclosure of which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

Technical Field

The present disclosure is directed to a method of preparing ametal/carbon nanocomposite and nanocomposite thin films made by themethod, the use of the nanocomposite films as plasmonic photocatalysts,in SERS-based sensors, as biomedical coatings, and for antimicrobialapplications such as water remediation. More specifically, thedisclosure is directed to an excimer laser irradiation process for thepreparation of textured Ag/C nanocomposite thin films.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Nanomaterials, such as nanocomposites, are multiphase solid materialswherein one of the phases has one, two or three dimensions of less than100 nanometers (nm), or structures having nano-scale repeat distancesbetween the different phases that make up the material. The propertiesof nanocomposites are determined not only by the morphology and spatialdistribution of the nanophase, but also depend on mutual chemical andphysical interactions between the various phases involved. Furthermore,a nanocomposite is any composite material, one or more, of whosecomponents are some form of nanoparticle. Nanoparticles, by definition,are particles between 1 nm and 100 nm in size. In nanotechnology, aparticle is defined as a small object that behaves as a whole unit withrespect to its transport and properties.

Metal nanoparticles, in particular, are recognized as being importantcontributors to the fields of chemistry, physics and biology due totheir unique optical, electrical and photo-thermal properties. Suchmetallic nanoparticles have great potential for application inlaboratory settings, as they can be used as probes in mass spectroscopy,and can also be employed in the colorimetric detection of proteins andDNA molecules. Metal nanoparticles have furthermore been used fortherapeutic applications and drug delivery. One metal of great interestis silver, in both nanoparticle and nanocomposite form.

Silver nanoparticle-based materials have recently gained great interestdue to their potential for use in a variety of fields, such as selectiveheterogeneous catalysis. Heterogeneous catalysis, in chemistry, refersto the form of catalysis where the phase of the catalyst differs fromthat of the reactants. This can include catalysis reactions such ashydrogenation, [B. J. Li, H. B. Li, Z. Xu, Experimental evidence for theinterface interaction in Ag/C60 nanocomposite catalyst and its crucialinfluence on catalytic performance. J. Phys. Chem. C 11 (2009)21526-21530 Incorporated by reference herein in its entirety],oxidation, [S. Y. Wu, Y. S. Ding, X. M. Zhang, H. O. Tang, L. Chen, B.X. Li. Structure and morphology controllable synthesis of Ag/carbonhybrid with ionic liquid as soft-template and their catalyticproperties. J. Solid State Chem. 2008, 181 (2008) 2171-2177.Incorporated by reference herein in its entirety], and hydrogen storage[S. Rather, M. Naik, S. W. Hwang, A. R. Kim, K. S. Nahm. Roomtemperature hydrogen uptake of carbon nanotubes promoted by silver metalcatalyst. J. Alloy Compd. 475 (2009) L17-L21 Incorporated herein byreference in its entirety].

Additionally, for many applications, the use of spherical nanoparticlesis desired. Methods are needed for consistent production of sphericalmetal nanoparticles for incorporation into nanocomposites. Moreover,there is also need for the development of cubic nanoparticles as well.Cubic nanoparticles, such as those incorporated into gold and silvernanocomposites, can be used to amplify the difference in left- andright-handed molecules' response to circularly polarized light. Severalstudies have indicate that these cubic nanoparticles provide the basisfor probing the effects of chirality, or handedness, in molecularinteractions with increased sensitivity over prior methods [Fang Lu, YeTian, Mingzhao Liu, Dong Su, Hui Zhang, Alexander O. Govorov, Oleg Gang,Discrete Nanocubes as Plasmonic Reporters of Molecular Chirality NanoLett., 2013, 13 (7), pp 3145-3151DOI: 10.1021/n1401107 g PublicationDate (Web): Jun. 18, 2013 American Chemical Society, Incorporated hereinby reference in its entirety]. There is a need for rapid methods ofsynthesizing both spherical and cubic silver nanoparticles, given theirwide range of applications.

In addition to gold/silver nanocomposites, silver nanoparticle-basedmaterials can incorporate other components, such as carbon, to form ananocomposite indicated by Ag/C which signifies a core/shell structure,in which Ag is the core and C forms the shell. Herein “/” indicates acore/shell form. In such form, Silver/Carbon nanocomposites have a widearray of applications, including catalysis.

Metal nanoparticles have proven effective as plasmonic photocatalystsdue to their surface plasmon resonance effects. Surface plasmonresonance (SPR) relates to the enhanced reflectivity of a dielectricmaterial. Recently, due to the SPR effect of silver nanoparticles,silver/carbon nanocomposites were reported as efficient plasmonicphotocatalysts due to their enhanced reflectivity under visible light[S. M. Sun, W. Z. Wang, L. Zhang, M. Shang, L. Wang. Ag/C core/shellnanocomposite as a highly efficient plasmonic photocatalyst. Catal.Commun. 11 (2009) 290-293. Incorporated herein by reference in itsentirety]. Additionally, Ag/C nanocomposites have exhibited highphotocatalytic activity in the decomposition of aqueous and gaseouscompounds under visible-light irradiation. The origin of this highphotocatalytic activity is mainly ascribed to the surface plasmonresonance (SPR) effect of silver nanoparticles in the Ag/C composite.Thus, these Ag/C nanocomposites are considered to be promising materialsfor use in solar light harvesting devices and sensors due to theirphotocatalytic activity.

In addition to the above applications, Ag/C nanostructures are importantfor biological applications due to the surface functional groupsprovided by the carbonaceous products [S. Li, X. Yan, Z. Yang, Y. Yang,X. Liu, J. Zou. Preparation and antibacterial property of silverdecorated carbon microspheres. Appl. Surf. Sci. 292 (2014) 480-487. Y.Zhao, Z. Q. Wang, X. Zhao, W. Li, S. X. Liu. Antibacterial action ofsilver-doped activated carbon prepared by vacuum impregnation. Appl.Surf. Sci. 266 (2013) 67-72. X. M. Sun, Y. D. Li. Ag/C core/shellstructured nanoparticles: controlled synthesis, characterization, andassembly. Langmuir 21 (2005) 6019-6024. Incorporated herein by referencein their entirety]. The antibacterial role of the Ag/C nanocomposites isdue to the toxicity of silver to microbes, and the use of silvernanoparticles is of interest because of the slower, more controlledrelease of silver ions. The slower release of silver cations, fromsilver nanoparticles and/or silver/carbon nanocomposites, can avoid theconstant delivery of an excess amount of silver to the area comparedwith other Ag⁺ based chemicals. Also, in using silver nanoparticles andnanocomposites, the metallic silver is not as susceptible todeactivation by a chloride molecule as compared with silver ions [Dunn,K.; Edwards-Jones, V. The role of Acticoat with nanocrystalline silverin the management of burns; Wythenshawe Hospital Burns Unit, Manchester,UK: England: United Kingdom, 2004; pp S1-9, Incorporated herein byreference in its entirety.]

There is a considerable interest and need for the advancement ofpreparation techniques for metal nanoparticle, metal/carbonnanocomposites, and specifically, Ag/C nanocomposites, also described ascore/shell nanocomposites. The preparation of metal/carbon nanocompositethin films, such as Ag/C nanocomposite thin films, wherein texturevariations result from differences in the molar ratio of carbon to themetal nanoparticle core is of further interest.

The preparation of silver and carbon core/shell composites has beenreported by both chemical and photochemical methods [S. M. Sun, W. Z.Wang, L. Zhang, M. Shang, L. Wang. Ag/C core/shell nanocomposite as ahighly efficient plasmonic photocatalyst. Catal. Commun. 11 (2009)290-293., X. M. Sun, Y. D. Li. Ag/C core/shell structured nanoparticles:controlled synthesis, characterization, and assembly. Langmuir 21 (2005)6019-6024, Incorporated herein by reference in their entirety.] However,these processes have disadvantages, in that they require significantlylengthy production times for synthesis, and the resulting silverparticle size distribution is not narrow. Furthermore, prior methods forforming silver carbon nanoparticles utilize surfactants and suffer fromissues with agglomeration, low volume production, and impurity.

The use of metal/carbon nanocomposites in the formation of thin films isan additional area of research for which there is considerable interestand need for advancement. Nanocomposite thin films possess improvedmechanical, electronic and magnetic properties as a result of severalfactors, such as crystallite size and textures. These factors dependsignificantly on the material selection, deposition methods, and processparameters in forming the nanocomposite thin films. Materials fornanocomposite thin films include, but are not limited to metals, such assilver, gold, palladium, nickel, cobalt, and non-metals such as carbonand nitrogen. By definition, thin film composites can include particles,both microparticles and nanoparticles, dispersed therein. Nanocompositefilms comprise at least two phases, a nanocrystalline phase and anamorphous phase, or alternatively, a nanocrystalline phase with anothernanocrystalline phase [S. Zhang, Y. Fu, H. Du, Y. Liu, T. Chen,Nanocomposite Thin Films for both Mechanical and Functional Applications2004 dspace.MIT-edu. Incorporated herein by reference in its entirety.]Due to their unique physical-chemical properties, nanocomposite thinfilms have recently gained interest for use in both the mechanical andfunctional fields, and furthermore, metal/carbon nanocomposites andnanocomposite thin films have been described as lubricants for solidsurfaces.

Techniques for preparation of nanocomposite films include, but are notlimited to, magnetron sputtering, chemical vapor deposition, and laserablation. In considering laser ablation methods, there are many types oflasers that can be employed; each operating at a defined wavelength oflight. In view of optical properties, excimer lasers possess advantagesover other types of lasers, including the Nd:YAG laser, in thin filmmanufacturing. The advantages are largely based on their superiorablation characteristics and much better energy stability. Majordrawbacks of the Nd:YAG lasers for pulsed laser deposition (PLD) includea Gaussian beam profile instead of a flat-top profile, as well astemperature-induced polarization and thermal lensing effects which cancreate donut-shaped beam profiles and lateral distortions. [Pulsed laserdeposition—UV laser sources and applications R. Delmdahl, R. PätzelCoherent GmbH, Hans-Böckler-Str. 12, D-37079 Göttingen, Germany PACS42.55.Lt, 52.38.Mf, 81.15.Gh Incorporated herein by reference in itsentirety.]

New preparation methods are sought to overcome disadvantages with theconventional preparation of metal/carbon nanoparticles and metal/carbonnanocomposite thin films. Accordingly, one objective of the presentdisclosure is to provide methods or processes of preparingnanocomposites and nanocomposite thin films.

BRIEF SUMMARY OF THE INVENTION

The present disclosure relates to a method and apparatus for thepreparation of uniformly sized and stable nanoparticles andnanocomposites, and their use to form nanocomposite thin films.

According to a first embodiment, the present invention is directed to amethod of preparing a core/shell nanocomposite thin film comprising:

concurrently irradiating a metal target and a hydrocarbon gas, presentwithin a deposition chamber, with an excimer laser beam;

wherein the irradiating forms carbon for example in the form of graphitefrom the hydrocarbon gas, and nanoparticles of said metal target, andconcurrently forms core/shell nanocomposite particles having a metalcore covered by a carbon shell;

wherein the core/shell nanocomposite particles form a nanocomposite thinfilm on a substrate within the deposition chamber;

wherein the pressure of the hydrocarbon gas in the deposition chamberduring the irradiating is within a range of 20-100 Pascals.

In a further embodiment, the nanocomposite thin film of a thickness inthe range of 200 nm to 1000 nm and texture in the range of 1 nm to 10 nmis formed within the time range of 30-90 seconds of the irradiating.

In a further embodiment, the hydrocarbon is selected from the groupconsisting of a C₁-C₁₀ alkane and a C₁-C₁₀ alkene.

In a preferred embodiment, the C₁-C₁₀ alkane is n-hexane.

In a further embodiment, the method further comprises varying thepressure of the hydrocarbon gas to result in a variation of a mass ratioof carbon to metal in the core/shell nanocomposite particles.

In a further embodiment, the irradiating forms core/shell nanocompositeparticles having an average particle size of 5-20 nm in diameter.

In a further embodiment, the nanoparticles are either spherical or cubicin form.

In a further embodiment, the metal is a transitional metal selected fromthe group consisting of Group 9, 10, or 11 transitional metals.

In a preferred embodiment, the transitional metal is silver.

In a further embodiment, the excimer laser beam is generated by anexcimer laser selected from the group consisting of ArF and KrF excimerlasers having a beam wavelength of 193 nm-248 nm.

In a preferred embodiment, the excimer laser is the ArF excimer laserwith a wavelength of 193 nm.

In a further embodiment, the core/shell nanocomposites formed by theirradiating have absorption peaks in a range from 417 nm-525 nm andfurther comprise a core of silver (Ag) and a shell of carbon (C).

In a further embodiment the disclosure relates to a thin film ofnanoparticles formed by the method of our first embodiment having athickness of 200 to 1000 nm.

In a further embodiment the disclosure relates to a thin film ofnanoparticles formed by the method of our first embodiment wherein theratio of carbon to silver varies from 1.10:1.00 to 1.75:1.00.

In a further embodiment the disclosure relates to a thin film ofnanoparticles formed by the method of our first embodiment whereinincreasing the pressure of the hydrocarbon gas from 20 Pa to 100 Paresults in a much thinner film.

In a further embodiment, a textured nanocomposite thin film is used insolar light harvesting devices.

In a further embodiment, a textured nanocomposite thin film is used inwater purification applications.

In a preferred embodiment, the water purification application isplasmonic photocatalysis.

In a further preferred embodiment, the water purification application isa nanocomposite thin film surface-enhanced Raman spectroscopy(“SERS”)—based sensor for the detection of contaminants in an aqueousphase.

In a further embodiment, a textured nanocomposite thin film is used as acoating for biomedical devices.

In a further preferred embodiment, the biomedical device is a stent.

According to a second embodiment, the present disclosure relates to amethod for forming a silver/carbon nanocomposite from a solid silvertarget during an excimer laser ablation process comprising:

providing a deposition chamber;

placing a silver metal target and a substrate within said depositionchamber;

establishing a vacuum level within said deposition chamber so as toachieve a reduced atmospheric pressure;

introducing a hydrocarbon into said deposition chamber wherein saidhydrocarbon is in a vapor phase due to the reduced atmospheric pressureand furthermore wherein said hydrocarbon vapor fills said depositionchamber and is in contact with said silver target;

focusing an excimer laser beam onto the silver target in contact withthe gaseous hydrocarbon at a power density high enough to release cubicor spherical uniformly-sized nanoparticles of the silver from the silversubstrate;

concurrently irradiating said hydrocarbon gas with the excimer laserbeam at a power density high enough to cause decomposition of thehydrocarbon gas;

said irradiation causing a carbonization of said silver nanoparticles toform a silver/carbon nanocomposite

-   -   collecting said silver/carbon nanocomposite on said substrate to        form a nanocomposite thin film;    -   wherein varying the chamber vacuum level results in fluctuation        of the hydrocarbon vapor pressure in the range of 20-100        Pascals, affecting the ratio of carbon to silver in the        silver/carbon nanocomposite, and furthermore resulting in        deposition texture variations of the nanocomposite thin film.

Therefore, the present disclosure relates to the preparation of a seriesof Ag/C nanocomposite textures with a controllable mass ratio of silverto carbon, via an excimer laser induced process, in the presence ofn-hexane as the carbonaceous precursor.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a schematic diagram of experimental setups for depositionof Ag/C nanocomposite textures.

FIG. 2 shows a schematic diagram of the optical emission spectrameasurement of a plume produced from silver target.

FIG. 3A shows SEM images and corresponding Energy-dispersive X-ray(“EDX”) results (inset) of as-deposited Ag/C composite textures preparedin the presence of n-hexane at pressures of 20 Pa.

FIG. 3B shows SEM images and corresponding Energy-dispersive X-ray(“EDX”) results (inset) of as-deposited Ag/C composite textures preparedin the presence of n-hexane at 60 Pa.

FIG. 3C shows SEM images and corresponding Energy-dispersive X-ray(“EDX”) results (inset) of as-deposited Ag/C composite textures preparedin the presence of n-hexane at 100 Pa.

FIG. 3D shows the molar ratios of carbon to silver at varying pressuresof n-hexane.

FIG. 4A shows TEM images and the corresponding SAED patterns (inset) ofas-deposited Ag/C nanocomposites in the presence of n-hexane atpressures of 20 Pa.

FIG. 4B show the phase results obtained from the corresponding selectedarea electron diffraction (“SAED”) patterns from FIG. 4A.

FIG. 4C shows the particle size distributions (sample size=50) obtainedfrom the corresponding selected area electron diffraction (“SAED”)patterns from FIG. 4A.

FIG. 5A shows TEM images and the corresponding SAED patterns (inset) ofas-deposited Ag/C nanocomposites in the presence of n-hexane atpressures of 60 Pa.

FIG. 5B show the phase results obtained from the corresponding selectedarea electron diffraction (“SAED”) patterns from FIG. 5A.

FIG. 5C shows the particle size distributions (sample size=50) obtainedfrom the corresponding selected area electron diffraction (“SAED”)patterns from FIG. 5A.

FIG. 6A shows TEM images and the corresponding SAED patterns (inset) ofas-deposited Ag/C nanocomposites in the presence of n-hexane atpressures of 100 Pa.

FIG. 6B show the phase results obtained from the corresponding selectedarea electron diffraction (“SAED”) patterns from FIG. 6A.

FIG. 6C shows the particle size distributions (sample size=50) obtainedfrom the corresponding selected area electron diffraction (“SAED”)patterns from FIG. 6A.

FIG. 7 shows FT-IR spectra of as-deposited Ag/C nanocomposite texturesin the presence of n-hexane at different pressures.

FIG. 8 shows Raman spectra of Ag/C nanocomposites deposited in thepresence of n-hexane at different pressures.

FIGS. 9A, 9B, 9C and 9D shows Optical emission spectra of ionized silverplasma generated by the ArF excimer laser at the distance of 0 mm awayfrom the target surface in the presence of n-hexane at differentpressures of 0 Pa (A), 20 Pa (B), 60 Pa (C) and 100 Pa (D),respectively.

FIGS. 10A, 10B, 10C and 10D shows Optical emission spectra of ionizedsilver plasma generated by the ArF excimer laser at the distance of 10mm away from the target surface in the presence of n-hexane at differentpressures of 0 Pa (A), 20 Pa (B), 60 Pa (C) and 100 Pa (D),respectively.

FIG. 11 shows UV-Vis spectra of Ag/C composite textures on quartzsubstrates deposited in the presence of n-hexane at pressures of 20 Pa,60 Pa and 100 Pa, respectively.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

The present disclosure pertains to a method for preparing silver/carbonnanocomposites. Herein the present disclosure combines the advantages ofperforming laser ablation of a metal target in a dry atmosphere, withthe simultaneous decomposition of a hydrocarbon, triggered by thecontrolled interaction of the laser energy, material and ambienthydrocarbon gas, thus forming a carbonized metal nanocomposite, andfurther a carbonized metal nanocomposite thin film.

One primary use for silver/carbon nanocomposites and composite films iswater remediation. Previously, suitable anchoring substrates were notavailable; therefore much of the silver nanoparticle or nanocompositecatalyst was lost to its undesirable release into the water. Therefore,prior water remediation has been hampered by the great difficulty ofusing catalysts in their powdered form for heterogeneous plasmonicphotocatalysis applications. Thin film preparation of a metal/carbonnanocomposite on a suitable anchoring substrate will result in thesimplification of the recycling process from an aqueous solution. Thinfilm preparation techniques, resulting in the rapid production of stablemetal/carbon nanocomposites for use in water remediation, is thereforenecessitated. Further water purification applications include theantibacterial properties of the silver ion, and the use of the Ag/Cnanocomposites as possessing antibacterial properties.

The Ag/C nanocomposites and nanocomposite thin films can be included inSERS based sensors for the detection of contaminants in an aqueousphase, such as water. As such, purification systems and waterpurification applications are greatly improved.

The Ag/C nanocomposites and nanocomposite thin films of the presentdisclosure have a role in biomedical devices. Medical devices orimplants that are in contact with body fluids or body tissues, such ascatheters, electrodes, and implants, are susceptible to immunereactions, formation of a thrombus, inflammation, and infection. Inorder to avoid these conditions, the surfaces of the biomedical devicescan be coated with the Ag/C nanocomposites and nanocomposite thin films,for example in a form immobilized on a surface, or covalently bondedwith biocompatible molecules. One application of the silver/carbonnanocomposite thin film is for use on tubular intravascular stents,which may comprise a radially expandable mesh type metal network. Thismetal network tends to be thrombogenic. It is within the scope of thisdisclosure to utilize specific coating parameters and fixturingtechniques in order to coat stents, and other medical devices andimplants, with a flexible and fully encapsulating Ag/C nanocompositefilm. The advantage of this biocompatible Ag/C thin film is increasedflexibility over traditional diamond like carbon films and the inclusionof silver for antimicrobial/antibacterial effects. More specifically, bycontrolling the deposition pressures during formation of thenanocomposite thin films, it is possible to produce biocompatiblenanocomposite thin films for use on stents or other related bio-medicalapplications with varying amounts of carbon to silver molar ratios.

Ag/C crystalline nanocomposite textures with controllable Ag/C molarratios have been developed using an excimer laser-induced ablationprocess and silver target under an alkane atmosphere, e.g., n-hexaneatmosphere.

Laser ablation has advantages in fabricating nanocomposites because alarge amount of atom clusters, or nanoparticles, are formed during theablation process. The shape of the nanoparticles ablated from the targetsurface material is also a significant and necessary characteristic indefining how a nanoparticle acts, interacts, or can be acted upon.Spherical and cubic particles are desirable for their uniform shape andrepeatable characteristics. Laser processing can form very high-qualitynanoscale particles of generally spheroid morphology and exceptionallytight particle size distribution. Additionally, these laser-inducedablations are very fast in comparison to magnetron sputtering andchemical vapor deposition methods.

In view of optical properties, the excimer laser, also known as anexciplex laser, possesses advantages over other types of lasers,including the Nd:YAG laser, in thin film manufacturing. These advantagesinclude superior ablation characteristics and much better energystability. An excimer laser is a form of ultraviolet laser which emits acool beam of ultraviolet light. The excimer laser typically uses amixture of a noble gas (argon, krypton, or xenon), and a halogen gassuch as fluorine or chlorine. When the laser pulse is absorbed by asurface material target, energy is first converted to electronicexcitation and then into thermal, chemical and mechanical energyresulting in evaporation, ablation, plasma formation and evenexfoliation. In a preferred embodiment, during application, an excimerlaser adds enough energy to disrupt the molecular bonds of the surfaceof the target material, resulting in the ejection of matter from thesurface of a solid metal target material in a tightly controlled mannerthrough ablation. At sufficiently high laser powers, the ejectedmaterial can be further excited to a plasma phase or plume, or an ejectaevent in a vacuum phase. In the present disclosure, the use of the termplume or ejecta event will signify the ablation of the target metal intoa stream comprising nanoparticles. Concurrently, the decomposition ofthe hydrocarbon into carbon and/or graphite takes place under excimerlaser beam irradiation. A laser ablative pulse gives rise to thedecomposition of the hydrocarbon gas into smaller molecules and ions, bythermal or photo-chemical decomposition. Excimer lasers have the usefulproperty that they can remove exceptionally fine layers of surfacematerial with almost no heating or change to the remainder of thematerial which is left intact, while concurrently interacting with agaseous environment to decompose said gaseous components into theiratomic constituents. Furthermore, excimer lasers provide the shortestcommercially available wavelengths and correspondingly, the highestcommercially available photon energies for laser material processing.Depending on the laser gas, the emission of excimer lasers covers arange of wavelengths from 157 nm to 351 nm. In conjunction with its highultraviolet output energies, an excimer laser ablation process offersgreat flexibility in terms of the target material spectrum and theablation area. Furthermore, the high energy photons of the excimer laserallow virtually all target materials to be deposited, including suchmaterials as oxides, nitrides, and carbides. Process stability and thinfilm quality in Pulsed Laser Deposition (PLD) procedures largely benefitfrom the exceptional pulse-to-pulse stability of the excimer laserradiation. The top-hat beam profile, which is imaged on the sample,minimizes fractionation during the ablation process and supportsstoichiometric transfer from the target material to a substrate, thusenabling a controlled thin film deposition.

In a preferred embodiment, an argon fluoride laser (ArF laser) is usedto carry out the method of this disclosure. It is a deep ultravioletexcimer laser, having an emission wavelength of 193 nm, and may be usedin the production of semiconductor integrated circuits, eye surgery,micromachining, and scientific research. Other excimer lasers, such asthe KrF laser, with a wavelength of 248 nm, may also be capable ofproducing the metal/carbon nanocomposite textures as well.

Pulsed laser deposition occurs when a high-power pulsed laser beam isfocused inside a vacuum chamber to strike a target of material whichdeposits to form a film. This material is vaporized from the target,which deposits as a thin film on a substrate. More specifically, theejected species expand into the surrounding vacuum in the form of anejecta plume or ejecta spray containing many energetic species includingatoms, molecules, electrons, ions, clusters, before depositing on asubstrate. This process typically occurs under reduced pressure, 100-900mTorr, for example, preferably 200-800 mTorr, or 300-500 mTorr. It mayalso take place in the presence of a background, or ‘assist’ gas, whichcan be used to provide a second target material, and reaction of the twotarget materials thus forming a nanocomposite. The byproducts of thelaser induced hydrocarbon decomposition process when combined with theablated metal species can form metal/carbon nanocomposites.

A solid target, which may be in the form of microparticles, is employedas the starting material in the production of nanoparticles, andnanocomposites. Microparticles can be composed of a wide variety ofmaterials of any shape. Preferably, such materials are solid at roomtemperature and are not susceptible to chemical degradation during thepractice of the present disclosure. Representative examples of suitablematerials include Group 9, 10, and 11 fransitional Metals such as gold,silver, palladium, platinum, nickel, iron and cobalt.

The first step in the creation of uniformly sized and cubic or sphericalshaped nanoparticles is ablation of a target surface to create a plumeor ejecta event which moves radially away from the surface of the targetmaterial. In a heavy atmosphere, i.e. a fluid medium, this ejecta eventis known as an ejecta plume which has a Knudsen boundary layerseparating the vapor within the plume (which contains the ejectamaterial) from the heavy atmosphere. In a vacuum, vaporization and/orablation of metal particles is preferably accomplished by delivering aspecific energy packet (typically photon or electric energy) to thetarget surface. This becomes phonon energy within the target surfacewhich is sufficient to break the intra-nuclear bonds around smallclusters of atoms, and further eject clusters away from the targetsurface at a rate which reduces residual heat within the target materialthat may lead to ion production. Altering the physical properties of thetarget material affects the ablation rate of that target. For instance,annealed metal targets, such as Ag, have reduced intra-nuclear bondingenergies and thus produce particles at higher rates from a constantenergy delivered.

Assist gasses can be used to modify the environment in which the laserprocess occurs. If a hydrocarbon gas is supplied to the laserinteraction zone, a decomposition reaction, ignited by the laser pulse,will result in the generation of carbon and carbonaceous products withpartial or complete graphite-like structures. The excimer laser strikesthe hydrocarbon vapor to decompose it and initiate particle nucleationin a reaction zone. Low molecular carbonaceous species can be producedby simultaneous dielectric breakdown of a hydrocarbon gas by the excimerlaser beam. Under certain conditions it may be possible to cause thegeneration of a liquid hydrocarbon phase at the surface of the metaltarget.

The hydrocarbon gases of this disclosure may be in a liquid form priorto vacuum treatment to render them into their gaseous phase.Additionally the hydrocarbon, when initially in a liquid phase, mayundergo distillation to remove impurities prior to placing it on avacuum line. It is within the scope of the present disclosure to usehydrocarbons such as methane, ethane and/or ethylene in this process.Representative examples of a suitable hydrocarbon include linear orbranched hydrocarbons, along with cyclic or aromatic hydrocarbons. In apreferred embodiment, the hydrocarbon is selected from the groupconsisting of short-medium- and long chained inert carbonaceous gases.In a further preferred embodiment, the hydrocarbon is a straight-chainedalkane or an alkene. In a most preferred embodiment, the hydrocarbon isselected from the group consisting of, but not limited to, C₃-C₁₀n-alkane or n-alkene hydrocarbons, which include n-propane, n-butane,n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane, aswell as n-propene, n-butene, n-pentene, n-hexene, n-heptene, n-octene,n-nonene and n-decene, and branched derivatives thereof. It is alsowithin the scope of this invention to deliver more than one alkane oralkene, or combination thereof, hydrocarbon gas into the depositionchamber. In a most preferred embodiment, the hydrocarbon is n-hexane.The approach in this invention is to deliver a hydrocarbon gas, orgases, into the deposition chamber, where a suitable combination of theexcimer laser, the metal material being irradiated, and thus ablated,the feature being formed by ablation, and pressure within the depositionchamber can be selected so as to form a nanocomposite thin film ofselected texture in the laser ablation process.

It is desirable to confine the hydrocarbon gas and the solid metalsubstrate target material so that they interact. This can be achieved byselecting a structure which suitably contains the hydrocarbon vapor andthe solid metal substrate target so that the excimer laser can interactwith both, and so that upon ablation, interaction of the hydrocarbongas, or decomposition products thereof, and the ablated plume of metalnanoparticles can ensue. Such confinement can occur in a depositionchamber such as a reactor as shown in FIG. 1, which may be spherical inform and may be made of amorphous glass. It is desirable that anappropriate mass flow rate of the hydrocarbon is maintained so that saidhydrocarbon remains in gas phase, that, when exposed to the excimerlaser beam, results in the formation of carbon atoms for carbonizationof the metal nanoparticles released by the solid metal target materialduring ablation with the excimer laser beam.

In a preferred embodiment, the pressure of the n-alkane (n-hexane) maybe adjusted such that the laser beam has interaction with thehydrocarbon gas at an optimal pressure within the range of 20-100Pascals. The molar ratio of carbon to silver can be controlled throughadjusting the n-alkane (n-hexane) pressure, producing spherical or cubicsilver nanoparticles with a size range of 5-20 nm and a relativelynarrow size distribution, thus providing a means of controlling thethickness and texture of the nanocomposite thin-film.

Reference is now made to FIG. 1 showing a representative laserdeposition system 106 for performing the method of the presentinvention. As shown, the experimental pulsed laser deposition system 106includes a vacuum deposition chamber 108 and a laser system (not shown).The vacuum deposition chamber is designed to confine all of thereactants of the laser ablation process. In a preferred embodiment, thedeposition chamber 108 is a spherical glass reactor having a diameter inthe range of 10-15 cm. The deposition chamber may contain severalwindows 114 for viewing and for the transmission of laser energy, onlyone of which is shown for clarity. A vacuum pump 124 is provided toevacuate the deposition chamber during the pulsed laser depositionprocess. As will be described in greater detail below, a pulsed laserbeam 110 is used to irradiate a target within the vacuum depositionchamber 108 in order to deposit ablated material(s) on a substrate 116.Further, one skilled in the art will recognize and understand that allmaterial surfaces within the chamber, input and output ports, tubing andstatic fixture materials are preferentially nonreactive, non-attractiveand non-absorbent to or with the specific nanoparticles being created.For example, untreated glass and quartz will readily absorb many typesof nanoparticles, particularly metallic particles and pose substantialproblems for use as materials for the reaction chamber. Preferredmaterials for carrying out the preferred method of this inventioninclude Teflon, PTFE, PEEK, and PET.

A primary excimer laser emits or delivers a discrete energy packet ofphoton energy in a pulsed manner. Laser beam 110 is generated by anexcimer laser (not shown). Any commercially available excimer lasercapable of generating a laser light in the ultra violet wavelengths inthe range from 157 nm to 351 nm can be employed. In one embodiment,irradiation conditions range from 1 s-120 s in time, with a repetitivefrequency of 5 to 15 Hz, and an energy range of 50 mJ/pulse to 100mJ/pulse. In a preferred embodiment, irradiation conditions range from55 s-65 s in time, with a repetitive frequency of 9 to 11 Hz, and anenergy range of 70 mJ/pulse to 90 mJ/pulse. In a most preferredembodiment, irradiation occurs for a 60 s time interval, with arepetitive frequency of 10 Hz, and an energy of 80 mJ/pulse.

The selection of a specific laser depends on the optical properties ofthe microparticles and the size of the nanoparticles desired. In apreferred embodiment, laser beam 110 with a wavelength range of 193-300nm, can be generated using an ArF excimer laser. In a most preferredembodiment, laser beam 110 with a wavelength of 193 nm is generatedusing an ArF excimer laser. Additionally, laser beam 110 can begenerated with a KrF excimer laser having a wavelength of 248 nm.

In order to carry out the method of the present invention, a target 120and a substrate 116 were placed within the deposition chamber 108. Thetarget 120 material can be any of a number of Group 9, 10, or 11transition metals such as silver, gold, platinum, nickel, cobalt orpalladium. In a preferred embodiment, the target material is silver. Thesubstrate 116 upon which the nanocomposite will be deposited is anon-reactive substance. Untreated quartz, amorphous glass and polishedNaCl will all readily absorb many types of nanoparticles, particularlymetallic nanoparticles and can be used as a surface substrate 116 forthe formation of the Ag/C nanoparticles, or other metal/carbonnanoparticles, and resulting films. In a preferred embodiment thesubstrate is amorphous glass. In an alternate preferred embodiment, thesubstrate is polished NaCl. Furthermore, the sample collector 116 ispositioned at an opposite side of the hydrocarbon gas inlet tube so thatthe sample collector is placed in the path of the laser-irradiated andtherefore, laser-ablated material.

According to an important aspect of the present invention, thedeposition chamber 108 is evacuated to a relatively moderate vacuumlevel, in the range of 20 to 100 Pascals. The hydrocarbon gas enters thedeposition chamber 108 via a gas inlet line 122. Flow controller 104 isa polytetrafluoroethylene (PTFE) stopcock valve used to restrict orisolate the flow of the gas thru the line of the aerosol feed source 122from a pressurized gas source (not shown). The hydrocarbon gas isintroduced into the deposition chamber 108 in a manner so that it reactswith the plasma ejecta plume 118 created when the target is irradiated,forming a nanocomposite material which is directed toward the substrate116 for deposition of the material thereon. The hydrocarbon gas flow isintroduced and delivered in the direction of the line through a line 122so that it purges other ambient gases from the deposition chamber priorto irradiating with the laser. As previously stated, line 122 isconnected to a source of hydrocarbon liquid (not shown) which isdistilled on a vacuum line and furthermore rendered into its gas phaseunder reduced pressure prior to entering line 122. In a furtherpreferred embodiment, the alkane hydrocarbon is n-hexane. Thehydrocarbon gas fills chamber so as to fill voids remaining after thetarget 120, substrate 116, and static supporting fixtures 100 and 102are placed within the deposition chamber. The hydrocarbon gas is foundunder these conditions within the deposition chamber 108 before anylaser reaction is initiated and takes place. The pressure is monitoredand controlled by a pressure sensor (not shown) and an equipped stopcockvalve 104. In a preferred embodiment the pressure sensor is an EdwardBarocel. In a preferred embodiment the stopcock valve 104 is made of anon-reactive material such that it does not collect any nanoparticulate,graphite or nanocomposites.

Laser beam 110 is introduced into the vacuum deposition chamber 108 atan angle of 180° which may vary by +/−5° after passing through aspherical converging lens 112 (f=30), the focal length being thedistance at which a beam of collimated light will be focused to a singlespot, and enters through a quartz glass window 114 before converging onthe target metal 120. Furthermore the beam 110 is focused on the surfaceof the metal target material 120 which is placed on a static fixture 102inside the deposition chamber 108. As the excimer laser (not shown)emits or delivers a discrete energy packet of photon energy via thelaser beam 110, laser irradiation and furthermore, ablation, takesplace. Once a pulse from emission 110 interacts with the surface of thetarget 120, the energy of the laser photons transfers into the latticestructure of the target 120 becoming a phonon. This energy breaks theintra-nuclear bonds within the lattice structure and releases particlesfrom the target 120 surface. This creates ablated particles within anejecta plume.

The utilization of a hydrocarbon atmosphere, as the laser emission 110interacts with the target 120, the ablated particles form an initialejecta plume containing discrete plume 118 to be deflected towards asubstrate on which the nanoparticles and or nanocomposites can form.During the process, nanoparticles are collected or deposited on asubstrate 116, said substrate held in place by a static fixture 100.

In order to facilitate continuous production and removal of the ablatedand carbonized particles, the chamber contains a gas inlet line 122 andevacuation output port 112 which are connected through input and outputtubing or piping or other similar structures to a tank or other similarholding vessels or chambers that contains the desired fluid, whetherliquid or gas or other heavy atmosphere. A mechanism for mixing thedesired fluid, whether by stirring or other mechanism may be included.The pressure in gas systems can be controlled by controlling the gaspressure. Similarly, in vacuum systems, the creation and maintenance ofthe vacuum within the system will operate with commonly understoodcomponents. The tank can further include a sample port (not shown) whichcould also include sensors for temperature, and/or fluid volume.

In a preferred embodiment, n-hexane underwent distillation on a vacuumline (not shown) prior to entering the deposition chamber 108 via thegas line 122. Flow was adjusted by flow controller PTFE stopcock 104 sothat n-hexane pressure was in the range of 20-100 Pascals, or theequivalent range of 150-750 mTorr. The pressure was monitored andcontrolled by an Edwards Barocel pressure sensor (not shown) andequipped with PTFE flow controller valve 104. According to an importantaspect of the invention, n-hexane underwent decomposition and dielectricbreakdown via an excimer laser to yield low molecular carbonaceousspecies. The carbonization process takes place adjacent to the silvermicroparticle mass. The simultaneous dielectric breakdown, ordecomposition, of n-hexane and the ablation of silver by the excimerlaser results in the formation of a low molecular weight carbonaceousspecies, along with spherical or cubic silver nanoparticles of arelatively small size distribution of 5-20 nm. The presence of thecarbonaceous species, preferably in a graphite form, allows theformation of a covering, or shell encompassing the silver nanoparticles,resulting in Ag/C nanocomposites which are crystalline in structure. TheAg/C molar ratios can be controlled via the variation of the pressure ofthe alkane. In particular the pressure of n-hexane adjusted within therange of 20-100 Pa, more specifically within a sub-range of 20-80 Pa,most specifically within the range of 50-70 Pa, influences the molarratio of carbon to silver in the nanocomposite and nanocomposite thinfilm. The lowest carbon to silver molar ratios were attained at thelower end of the pressure range (i.e. 20 Pa). The ratio at 20 Pa wasfound to be 1.10:1.00. The carbon:silver molar ratio was found toincrease with increasing pressures to a maximum of 1.75:1.00 at apressure of 60 Pa. An increase of pressure beyond 60 Pa to 100 Paresulted in a similar, yet slightly lower carbon:silver molar ratio of1.65:1.00. Concurrent with this was the formation of thicker films. Atlower pressures (20-60 Pa) a thicker film (up to 1000 nm) was obtainedon the substrate, while at the increased pressure of 100 Pa, a muchthinner film was achieved.

Vacuum was maintained in the deposition chamber by an evacuation line124. The deposition chamber was evacuated to a relatively moderatevacuum level of between 20 and 100 Pascals of the hydrocarbon gasn-hexane. The target 120 metal, silver (Ag) was then irradiated withpulsed light laser beam 110 emanating from the laser (not shown). Theinteraction of the light from the laser and the target causes ejectionof atomic and molecular species from the silver target 120, resulted inplume 118. By virtue of the relatively moderate vacuum level, ejectedmaterial is slowed by collisions with the background n-hexane gas, whichin itself was decomposed by said laser beam when striking thehydrocarbon. This allows the ejected atoms and molecules from the targetmaterial to recombine with the decomposition graphite products in thegas phase to effectively produce nanocomposites having a core silvermetal and a hydrocarbon and/or carbon and/or graphite shell. Thenanocomposites then collect on a substrate 116 held in place inside thedeposition chamber 108 by a static fixture 100. The laser irradiationcontinues for a time sufficient to collect the desired layer ofnanocomposite on the substrate 116. In this way, the nanocomposite is acore/shell composite of the silver target nanoparticles, and the carbonfrom the hydrocarbon gas. The layer of silver/carbon nanocomposite filmdeposited on the substrate may vary in texture due to the variation ofpressure of said hydrocarbon gas, and the time the nanoparticles spendin the laser interaction zone. One skilled in the art will also notethat temperature may also affect the texture of said nanocomposite thinfilm and varying the temperature so as to affect the texture of thenanocomposite thin film is also considered within the scope of thisinvention. In the preferred embodiment, parameters for the laser, suchas pulse energy, repetition rate, pulse duration, wavelength, and beamsize, are first set to optimize laser ablation. Adjustments are thenmade to optimize the generation of carbonization of the hydrocarbon gasso that the carbon to metal ratio in the resulting nanocomposites andnanocomposite films results in desirable nanocomposite thin filmtextures. In one embodiment the texture of the film, when measured innm, refers to the average height of protrusions from the surface of thefilm.

As mentioned above, adjustments may also be made to assure transport ofthe irradiated, and thus ablated, metal away from the laser interactionzone, such as through the use of confinement and deflection magnetics.This step is important in influencing the overall efficiency and/oramount of carbon deposited on the metal nanoparticles during the laserablation process, as it can avoid or encourage re-deposition ofmaterials back into the laser interaction zone, thus affecting thecore/shell ratio by influencing the graphite deposition on the metalnanoparticles. However, an increase in material found in the laserinteraction zone conversely reduces the overall process efficiency ofthe laser process.

Referring now to FIG. 2, there is depicted a schematic spectroscopic setup used for the optical emission (OE) investigation of the plume 118. Aspectrophotometer system 130, including a photometer 128, and a detector126, is integrated into the laser ablation setup to record the opticalemission (OE) spectra of ablated materials collected from the metaltarget at various distances from the said target surface. The photometer128 used to determine the optical properties of said plume may include aUV-vis spectrometer, such as, but not limited to a Shimadzu model. Thephotometer 128 was used to confirm the release of ionized metal, as wellas electrons, from the surface of the target metal in a hydrocarbonatmosphere.

Further to the characterization of the nanomaterials is an analysis bySEM-EDX, TEM-EDX, SAED, FTIR, Raman and UV-Vis absorption analysis, asfurther discussed below.

The examples below are intended to further illustrate protocols forpreparing and characterizing the various embodiments of metal/carbonnanocomposites and nanocomposite thin films described herein, and arenot intended to limit the scope of the claims.

Example 1

Ablation of Silver Microparticles in a n-Hexane Vacuum to Form Ag/CNanocomposites and Texturized Nanocomposite Thin Film: Using theapparatus as shown schematically in FIG. 1, silver nanoparticles weredispersed into an argon stream after a deposition chamber was evacuatedto a pressure of 20, 60 or 100 Pascals n-hexane (after distillation on avacuum line). The pressure of the n-hexane was monitored and controlledby an Edward Barocel pressure sensor (not shown) and a flow line 122equipped with a PTFE valve 104, respectively. A silver target wasirradiated with the output of an ArF 193 nm excimer laser beam as thesource of excitation, for a collective irradiation period of 60 seconds.The solid microparticles that were ablated by irradiation resulted insilver nanoparticles. These silver nanoparticles interacted with carbon,in the form of graphite, concurrently released from the decomposition ofn-hexane gas, thus forming a silver core and graphite shell (Ag/C)nanocomposite. These nanocomposites were collected downstream from thelaser by an amorphous glass substrate 116 positioned at an edge of thespherical glass deposition chamber 108 (diameter=13 cm) opposite thehydrocarbon flow line 122. The as-prepared Ag/C nanocomposite texturewas named according to the preparation conditions as illustrated by“Ag/C-x”, and wherein “x” (x=20, 60 and 100) was the pressure ofn-hexane gas (in units of Pascal) filled inside the deposition chamber108 before irradiation.

Ag/C nanocomposite thin films were directly deposited on polished NaClsubstrates for FT-IR measurements, on quartz substrates during UV-Visspectra analysis, and on Beryllium substrates for EDX analysis; allunder identical experimental conditions as compared to the initialcollection of said thin films.

Determining the size distribution of the nanoparticles generated inaccordance with the practice of this invention typically requires alarge number of individual nanoparticles so that a statisticallysignificant distribution can be obtained. Herein, all measurements wereconducted twice and the average value was adopted in the process ofcollecting the nanoparticles disclosed herein. A scanning electronmicroscope (SEM) can be used to obtain photomicrographs ofnanoparticles. For instance, nanoparticles can be deposited on a quartzsubstrate and the surface of the quartz substrate can subsequently bescanned to photograph the nanoparticles. In addition, it is well knownin the art to employ transmission electron microscopy to obtain imagesof the nanoparticles. The selected area electron diffraction (“SAED”)technique is a crystallographic experimental technique that can beperformed inside a transmission electron microscope. As a diffractiontechnique, it can be used to identify crystal structures and examinecrystal defects.

Further techniques include energy-dispersive X-ray spectroscopy (“EDX”),a useful analytical technique for the elemental analysis or chemicalcharacterization of a sample, as it allows for the elemental compositionof a sample specimen to be measured via the number and energy of theX-rays emitted form a specimen. Fourier transform infrared spectroscopyis a measurement technique is used to obtain an infrared spectrum ofabsorption, emission, photoconductivity or Raman scattering of a solid,liquid, or gas. This can be used to identify the decomposition productsof n-hexane by ArF excimer laser, as well as the products of thetarget/hydrocarbon interaction.

Raman spectroscopy is a spectroscopic technique used to observelow-frequency modes in a system. It relies on inelastic scattering ofmonochromatic light, usually from a laser in the visible, near infrared,or near ultraviolet range. Surface plasmon resonance (“SPR”) is theresonant oscillation of conducting electrons at the interface between anegative and positive permittivity material stimulated by incidentlight. SPR is the basis of many standard tools for measuring adsorptionof material onto planar metal surfaces, or onto the surface of metalnanoparticles.

Characterization Methods

The transmission electron microscopy (TEM) images, including selectedarea electron diffraction (SAED) patterns of as-deposited Ag/Cnanocomposites, were recorded on a transmission electron microscope(JSM-6510). Fourier transform infrared spectra (FT-IR) of Ag/Cnanocomposites deposited on polished NaCl substrates were recorded atroom temperature using a FTIR spectrophotometer (Impact 400, ThermoNicolet), in order to identify the decomposition products of n-hexaneinduced by ArF excimer laser.

The decomposition products of carbon that converted from n-hexane underirradiation of ArF excimer laser were further investigated using laserRaman spectrometer (λ=473 nm, 30% of Power). The deposited solidtextures were carefully transferred to a polished tungsten substrate byscratching before the laser Raman spectroscopy measurement. FIG. 2depicts the acquisition/deposition system 106, spectrometer, 126 anddetector 128 of the spectroscopic setup that was used for the opticalemission (OE) investigation of the ablation plume. The optical propertyof as-deposited Ag/C nanocomposite textures on quartz was measured by aUV-Vis spectrometer (Shimadzu). The complete graphite-like structure,—C═C— group and sp3 C—H stretch vibration were observed from Raman andF-IR measurements. The remarkable decrease in optical intensity of thesilver plume by increasing the pressure of n-hexane from 60-100 Pa wasfound, which could be the main reason responsible for the much thinnerlayer deposited at pressure of 100 Pa. The dependent of SPR position(red-shift amount) on the silver nanoparticles concentration was alsonoticed and such red shift is deduced to be caused by the mutualpolarization of the particles.

SEM-EDX Analysis

The morphology of as-deposited Ag/C textures was investigated using SEMand the corresponding composition analysis was carried out as well (asdepicted in FIG. 3). EDX analysis that was measured on a berylliumsubstrate suggests the existence of carbon and silver elements. Thepressure of n-hexane, the main source of carbon, was the greatestinfluence on the amount of carbon deposited on the substrate. Theproduct yield of carbon, as well as the molar ratio of carbon to silver,was found to increase with the increase in pressure of n-hexane up to 60Pa. At that pressure, the molar ratio of carbon to silver reaches1.75:1. A further increase in pressure of n-hexane above 60 Pa did notyield further increases in product yield of carbon nor improve the ratioof carbon to silver, thus indicating that the saturation of carbon yieldwas most likely reached by a pressure of n-hexane at 60 Pa. In suchsituation, it is understood that virtually all the photons from the ArFexcimer laser have contributed in the conversion of n-hexane to carbon.

TEM-EDX Analysis

TEM and SAED were carried out to further investigate the morphology andthe crystal structures of metallic silver in Ag/C composites depositedin the presence of n-hexane in the 20-100 Pa pressure range. As clearfrom FIG. 4, spherical nanoparticles with the size of 5-20 nm andrelatively narrow size distribution can be clearly observed from therepresentative TEM images of Ag/C-20 and Ag/C-60. The position andrelative intensity of scattering vectors calculated from the SAEDpattern, could be well indexed as the existence of cubic silver (JCPDSNo. 04-0783), demonstrating the cubic metallic silver were deposited onthe substrate after being atomized from the target under excimer laserirradiation.

FT-IR Analysis

The Ag/C nanocomposite textures were directly deposited on polished NaClsubstrates under identical experimental conditions for FT-IRmeasurement. As depicted in FIG. 5, no peak could be distinguished fromthe FT-IR spectrum of the Ag/C-100 sample due to the small thickness ofas-deposited sample, except the absorption at 2360 cm⁻¹ which isattributed to the CO₂ asymmetric stretch vibration (not shown). Thestretching mode of C₂ can be observed at wavenumbers 1562 and 1593 cm⁻¹for the samples Ag/C-20 and Ag/C-60, respectively. Moreover, the sp³ C—Hstretch vibration can be observed at wavenumbers centered at2861/2924/2958 cm⁻¹ and 2918 cm⁻¹ for the samples Ag/C-20 and Ag/C-60,respectively. The FT-IR results are aligned with the findings of theRaman analysis.

Raman Analysis

The structure of carbonaceous products converted from n-Hexane by ArFexcimer laser was further studied by means of Raman characterization. Itshould be noticed that only 30% of the incident laser power was utilizedduring the Raman spectra measurement in order to prevent unnecessarydamage (oxidation) of the samples. As depicted in FIG. 6, all of theAg/C nanocomposite samples display two Raman peaks centered at ˜1366cm⁻¹ and 1585 cm⁻¹, which correspond to the D band and G bandscatterings, respectively. Compared to the broad D band observed, the Gband, which is caused by the first-order scattering of the E_(2g) modeobserved from sp² carbon domains, is much sharper and has far higherRaman intensity, demonstrating the existence of carbonaceous productswith complete graphite-like structures in the Ag/C nanocomposites.

Optical Emission Spectra During Laser Ablation Process

FIG. 7 depicts the optical emission spectra collected from the targetsurface level (d=0 cm) at a distance of 1 cm from the target surface. Itcan be found that most of the lines exhibited in these spectra could beattributed to the ionized silver. In both cases, the strong linescentered at 519 and 545 nm are identified as the silver transitions of4d¹⁰5p-4d¹⁰5d. [A. Striganove, N. Sventiski, Table of Spectral Lines ofNeutral and Ionized Atoms, Plenum Press, New York, 1968 Incorporated byreference herein in its entirety.] The strongest line at approximately385 nm is caused by the second harmonic from the ArF excimer laser. Fromthe optical emission spectra series measured at the surface of target,no apparent changes in the emission intensity in the UV-Vis region wereobserved in the presence of n-hexane at varying pressures. However, thenegative effect of excessive n-hexane on the emission intensity detectedat the distance of 10 mm from the target can be visibly discerned inFIG. 7. The OE intensity apparently does not change in the presence ofn-hexane at pressures varying from 20 Pa to 60 Pa. Nevertheless, theoptical emission intensities at both wavelengths, 518 nm and 546 nm,decrease almost two fold after filling n-hexane in the 60 Pa to 100 Papressure range. The ionization energy of silver atoms and the photonenergy of the ArF excimer laser (λ=193 nm) were 7.57623 eV and 6.42 eVrespectively. Thus approximately 1.2 photons are needed for theionization of one silver atom from the target, and concurrently, anelectron will be emitted from the target as well. The lower pressurefrom 20 Pa to 60 Pa of n-hexane may lead to higher emission speed forelectrons. This facilitates the recombination of electrons and ions oratoms; therefore, a much thicker texture is obtained. In contrast, afurther increase in the pressure of n-hexane, for example to 100 Pa,inhibits the emission speed of electrons, giving rise to thinner films.

UV-Vis Absorption Analysis

Silver particles can be identified by UV-Vis absorption measurements, assilver particles exhibit an intense absorption band caused by thesurface plasmonic excitation. As depicted in FIG. 8, the absorptionbands in the visible region suggest that silver nanoparticles areproduced. The absorption peak of the Ag/C composites deposited in thepresence of n-hexane at 20 Pa is centered at 519 nm. The small red-shiftto 471 nm (˜50 nm of shift) can be found when increasing the pressure ofn-hexane to 60 Pa (increasing the carbon/silver molar ratios from 1.1 to1.75. Furthermore, the Ag/C-100 sample which has a similar molar ratioof carbon/silver compared with sample of Ag/C-60, exhibits a similarsurface Plasmon resonance (SPR) position centered at approximately 525nm. These results clearly suggest that the SPR position is stronglydependent on the concentration of metal (silver) nanoparticles in thecomposites, and is most likely caused by the mutual polarizations of theparticles. Conclusively, the method of producing metal/carbonnanocomposites and nanocomposite thin films with variations in texturewas carried out within a relatively short time frame of 60 seconds, andproduced the largest molar ratio of carbon graphite to silvernanoparticles at a n-hexane pressure of 60 Pascals.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

The invention claimed is:
 1. A method of preparing a core/shellnanocomposite thin film comprising: concurrently irradiating a silvermetal target and n-hexane gas, present within a deposition chamber, withan excimer laser beam; wherein the irradiating forms carbon in the formof graphite from the n-hexane gas, and silver nanoparticles of saidsilver metal target which are spherical or cubic in form, therebyforming core/shell nanocomposite particles having a silver nanoparticlecore covered by a graphite shell; wherein the core/shell nanocompositeparticles form a nanocomposite thin film on a substrate within thedeposition chamber; wherein the pressure of the n-hexane gas in thedeposition chamber during the irradiating is within a range of 20-100Pascal.
 2. The method of claim 1, which forms the nanocomposite thinfilm within 30-90 seconds.
 3. The method of claim 1, further comprisingvarying the pressure of the n-hexane gas to vary a mass ratio of carbonto metal in the core/shell nanocomposite particles.
 4. The method ofclaim 1, wherein the irradiating forms the core/shell nanocompositeparticles having an average particle size of 5-20 nm in diameter.
 5. Themethod of claim 1, wherein the excimer laser beam is generated by anexcimer laser selected from the group consisting of ArF and KrF excimerlasers having a beam wavelength of 193 nm-300 nm.
 6. The method of claim5, wherein the excimer laser is an ArF excimer laser with a wavelengthof 193 nm.
 7. The method of claim 1, wherein the core/shellnanocomposite particles have absorption peaks in a range from 417 nm-525nm.
 8. The method of claim 1, wherein the nanocomposite thin film formsa coating for a biomedical device.
 9. The method of claim 8, wherein thebiomedical device is a stent.
 10. The method of claim 1, wherein thecore/shell nanocomposite thin film forms a coating for a solar lightharvesting device or sensor.
 11. The method of claim 1, wherein thecore/shell nanocomposite thin film is present on a surface of a waterpurification apparatus.
 12. The method of claim 11, wherein the waterpurification apparatus has a plasmonic photocatalysis surface.
 13. Themethod of claim 11, wherein the water purification apparatus includes ananocomposite thin film surface-enhanced Raman spectroscopy sensor. 14.A method for forming a silver/carbon nanocomposite from a solid silvermetal target during an excimer laser ablation process comprising:providing a deposition chamber; placing the solid silver metal targetand a substrate within said deposition chamber; establishing a vacuumlevel within said deposition chamber so as to achieve a reducedatmospheric pressure; introducing n-hexane into said deposition chamberwherein said n-hexane is in a vapor phase due to the reduced atmosphericpressure and furthermore wherein said n-hexane vapor fills saiddeposition chamber and is in contact with said solid silver metaltarget; focusing an excimer laser beam onto the solid silver metaltarget in contact with the n-hexane vapor at a power density high enoughto release cubic or spherical uniformly-sized silver nanoparticles fromthe solid silver metal substrate; concurrently irradiating said n-hexanevapor with the excimer laser beam at a power density high enough tocause decomposition of the n-hexane vapor; said irradiation causing acarbonization, in the form of graphite, of said silver nanoparticles toform core/shell nanocomposite particles having a silver nanoparticlecore covered by a graphite shell; collecting said core/shellnanocomposite particles on said substrate to form a core/shellnanocomposite thin film; wherein varying the chamber vacuum levelresults in fluctuation of the n-hexane vapor pressure in the range of20-100 Pascal, affecting the ratio of carbon to silver in thesilver/carbon nanocomposite, and resulting in deposition texturevariations of the core/shell nanocomposite thin film.